The present invention relates to turbomachinery, and more particularly to anti-icing and de-icing systems for aircraft engine nacelles fabricated at least in part from composite materials.
High-bypass turbofan engines are widely used for high performance aircraft that operate at subsonic speeds. As schematically represented in FIG. 1, a high-bypass turbofan engine 10 includes a large fan 12 placed at the front of the engine 10 to produce greater thrust and reduce specific fuel consumption. The fan 12 serves to compress incoming air 14, a portion of which flows into a core engine (gas turbine) 16 that includes a compressor section 18 containing low and high pressure compressor stages 18A and 18B to further compress the air, a combustion chamber 20 where fuel is mixed with the compressed air and combusted, and a turbine section 22 where a high pressure turbine 22A extracts energy from the combustion gases to drive the high pressure stages 18B of the compressor section 18 and a low pressure turbine 22B extracts energy from the combustion gases to drive the fan 12 and the low pressure stages 18A of the compressor section 18. A larger portion of the air that enters the fan 12 is bypassed to the rear of the engine 10 to generate additional engine thrust. The bypassed air passes through an annular-shaped bypass duct 24 that contains one or more rows of stator vanes, also called outlet guide vanes 28 (OGVs), located immediately aft of the fan 12 and its fan blades 26. The fan blades 26 are surrounded by a fan cowling or nacelle 30 that defines the inlet duct 32 to the turbofan engine 10 as well as a fan nozzle 34 for the bypassed air exiting the bypass duct 24.
The nacelle 30 is an important structural component whose design considerations include aerodynamic criteria as well as the ability to withstand foreign object damage (FOD). For these reasons, it is important to select appropriate constructions, materials and assembly methods when manufacturing the nacelle 30. Various materials and configurations have been considered, with metallic materials and particularly aluminum alloys being widely used. Composite materials have also been considered, such as graphite-reinforced epoxy laminates, as they offer advantages including the ability to be fabricated as single-piece parts of sufficient size to meet aerodynamic criteria, contour control, and reduced weight, which promote engine efficiency and improve specific fuel consumption (SFC).
Aircraft engine nacelles are subject to icing conditions, particularly the nacelle leading edge at the inlet lip (36 of FIG. 1) while the engine is on the ground and especially under flight conditions. One well known approach to removing ice buildup (de-icing) and preventing ice buildup (anti-icing) on the nacelle inlet lip has been through the use of hot air bleed systems. An example is schematically represented in FIG. 1, in which engine-supplied bleed air flow is drawn from the combustion chamber 20 through piping 38 to the inlet lip 36, where the hot bleed air contacts the internal surface of the inlet lip 36 to heat the lip 36 and remove/prevent ice formation. The piping 38 includes a tube arrangement commonly referred to as a piccolo tube 40, which resides in an annular-shaped cavity of the nacelle 30 sometimes referred to as the D-duct 42. The tube 40 completely fills the D-duct 42 with the hot bleed air to ensure adequate heating of the inlet lip 36. While this type of system is effective, it requires a large amount of bleed air to fill the D-duct 42 and provide the thermal energy necessary to perform the anti-icing function. The hot air bled from the engine 10 results in a corresponding negative impact on engine performance and detracts from engine efficiency (SFC). Additionally, hot air bleed systems of the type represented can incur a significant weight penalty.
As an alternative, some smaller turbofans and turboprop aircraft engines have utilized electrical anti-icing systems that convert electrical energy into heat via Joule heating. Resistance-type heater wires can be used as the heating element, though a more recent example uses a flexible graphite material commercially available under the name GRAFOIL® from GrafTech International Holdings Inc. The heating element is embedded in a boot, such as a silicon rubber, which in turn is attached to the inside leading edge of the nacelle inlet lip. A drawback of such systems is that they typically require excessive energy for de-icing and continuous anti-icing operation on large aircraft engines, such as high-bypass turbofan engines of the type represented in FIG. 1. Furthermore, electrical anti-icing systems are relatively heavy and detract from engine efficiency/performance.
Still other options include “weeping” systems that release chemical de-icing agents, and de-icing boots equipped with inflatable bladders to crack ice buildup. Notable disadvantages of weeping systems include the high cost of chemical de-icing agents, the requirement that the aircraft carry the de-icing agent at all times, and the inoperability of the system if the supply of chemical agent is exhausted during flight. Disadvantages of de-icing boots include the requirement for a pump to inflate the bladders and a relatively short life span.
In view of the above, there are ongoing efforts to develop new technologies capable of providing de-icing and anti-icing functions while minimizing any negative impact on weight and power requirements, particularly with regard to the use of nacelles and other airfoil surfaces that are fabricated from composite materials to promote overall engine performance. However, the use of composite materials such as graphite-reinforced epoxy laminates in place of conventional aluminum alloy nacelles and wing structures poses additional challenges to anti-icing systems, since laminate composite materials exhibit relatively poor thermal conductivity in the thickness direction (and therefore between adjacent laminae), reducing the efficiency with which conventional anti-icing systems can heat the outer surfaces of a nacelle to remove and prevent ice buildup.